A large number of sheet switches such as a sheet switch 10 shown in FIG. 11 are conventionally used as key input parts provided in various electronic instruments such as mobile phones, mobile information terminal devices or the like, and have illumination function by positioning light emitting elements 6 as light sources disposed about key switches 8. On the other hand, as shown in FIG. 12, there is known a sheet switch 1 having a structure in which the light emitting elements 6 and a flat plate-like light guiding plate 5 are combined (see Patent Document 1). The sheet switch 1 includes a circuit board 2 provided with fixed contacts 3 and wiring patterns, movable contacts 4 each disposed over each of the fixed contacts 3 and further including tactile springs, a light guiding plate 5 disposed above the circuit board 2, light emitting elements 6 to illuminate one side surface of the light guiding plate 5 and key tops 7 to press the movable contacts 4 respectively, and lights emitted from the light emitting elements 6 are transmitted through the light guiding plate 5. In the sheet switch 1, all the key tops 7 are illuminated by irradiating light guided from the light emitting elements 6 to lower portions of the key tops 7 passing through the light guiding plate 5.
On the other hand, to correspond to recent thinned electronic devices, further thinned sheet switches with illumination function are known (see Patent Document 2). FIG. 13 illustrates a sectional structure of a sheet switch 11 as described in Patent Document 2. The sheet switch 11 includes a key switch 18 having a circuit board 12 provided with fixed contacts 13 and movable contacts 14 disposed over the fixed contacts 13, a light guiding plate 15 covering the key switch 18, and key tops 17 disposed in upward positions of the light guiding plate 15 where the movable contacts 14 are provided. In the sheet switch 11, the entirety of the light guiding plate 15 is lighted and the key tops 17 are illuminated from lower surfaces thereof, by irradiating light emitted from the light emitting elements 16 arranged at one end of the circuit board 12 to the side surface of the light guiding plate 15.
Patent Document 1: Japanese Patent Laid-Open No. 2004-69751 (FIG. 9)
Patent Document 2: Japanese Patent Laid-Open No. 2004-69751 (FIG. 6)